I. End-to-end Communication
In a typical wireless communication system, a mobile station may digitize and encode an analog voice signal received from a user for transmission over an air interface to a radio access network. A base station controller (“BSC”) within the radio access network may decode the encoded voice signal and send a digital representation of the voice signal to a network switch for transmission to another endpoint.
Similarly, the radio access network can receive a digital representation of a voice signal destined for a mobile station. The BSC within the radio access network would then encode the digital representation for transmission over the air interface to the mobile station. The mobile station may then decode the encoded voice, convert the underlying digital representation to an analog signal, and output the analog signal to a user.
When a voice call is placed between two mobile stations, the system is inherently inefficient, because the system will unnecessarily perform back-to-back decoding and encoding of data. Namely, a sending mobile station would encode a voice signal for transmission to an originating BSC. The originating BSC then decodes the voice signal sent from the mobile station, and sends the decoded signal through the radio access network to a receiving BSC, which could be the same as the original BSC. The receiving BSC would then encode the voice signal and transmit it to a receiving mobile station, which would then decode the voice signal. This back-to-back “vocoding” (voice decoding and/or encoding) can degrade signal quality and delay voice transmission.
II. Vocoder Bypass
One mechanism that can be used to overcome this problem is known as “tandem free operation” (“TFO”) or “vocoder bypass.” According to this mechanism, if a voice call is a mobile-to-mobile call (i.e., between mobile stations), then data may be passed transparently through the network between the mobile stations without back-to-back vocoding. On the other hand, if the voice call is between a mobile station and a non-mobile station, then the data may be vocoded for transmission to a switch serving the non-mobile station.
In processing a call, the network can determine whether a given call is mobile-to-mobile by looking up telephone numbers of the endpoints of the call in a database that correlates the telephone numbers with device types. If the database indicates that the telephone numbers for both terminating endpoints of a call correspond to mobile stations, the network may conclude that the call is a mobile-to-mobile call and could then perform vocoder bypass.
However, existing methods for determining whether to perform vocoder bypass may have a number of disadvantages. For example, such methods can have relatively long delays associated with looking up telephone numbers in a database. The time required to search such a database may be proportional to the size of the database. Thus, as a greater number of users begin using mobile devices, the size of the database may increase, and delays associated with looking up identifiers in the database may grow over time.
Additionally, looking up identifiers may complicate the process of making a call by adding another step, which may result in more dropped calls and a degraded signal quality. Furthermore, vocoder bypass is generally not supported between mobile stations using different wireless service providers. Thus, the use of vocoder bypass may be severely limited in a multi-carrier market.